The present invention relates to the field of current measurement, particularly in the context of pulse-width-modulated (PWM) circuits. More specifically, the invention relates to a circuit which provides accurate measurement of current flowing through a load for a dual totem (H-bridge) power stage, while maintaining galvanic isolation between the measurement circuit and the load.
Examples of PWM circuits are shown in U.S. Pat. Nos. 5,070,292, 5,081,409, 5,379,209, and 5,365,422. The disclosures of these patents are hereby incorporated by reference. These patents provide examples of circuits in which a series of pulses is used to control electronic switches which selectively connect a power supply to a load. The load can be an electric motor, or a coil used to produce a magnetic field, or some other load.
In PWM circuits of the types described in the above-cited patents, it is often necessary to monitor the current flowing through the load, either for purposes of overcurrent protection, or to control another circuit based on the measured current in the load, or for other reasons. Direct measurement of load current is undesirable because it requires the insertion of an inductance or a resistance into the circuit being monitored. Preferably, the current measurement technique will maintain galvanic isolation, i.e. insuring that no current flows directly between the load and the measuring circuit.
However, in the prior art, there are few techniques for measuring load current in a PWM circuit while maintaining galvanic isolation. While the load can be coupled, through a transformer, to a conventional circuit for current measurement, the accumulation of magnetic flux in the transformer core accentuates the nonlinearity of the transformer and introduces inaccuracy into the final measurement. A solution to this problem is to use a larger transformer, which is less likely to experience core saturation and which therefore provides a greater range over which the transformer response is relatively linear. However, using a larger transformer has the disadvantage of requiring a larger space, and it may also be unacceptably expensive.
In some current measurement circuits, during times in which the sensed load current is changing in response to PWM control signals, the output of the current measurement circuit may not represent the actual load current with the level of accuracy desired. For example, in some current measurement circuits, the load current indicative output can be erroneous by an amount proportional to the rate of change of the load current.
In prior art current measurement circuits for measuring the load current through a dual totem power stage (an H-bridge), two transformers were employed, one for each totem of the H-bridge. Likewise, the prior art required two flux balance circuits and two decommutation circuits, one for each totem. This presented numerous disadvantages, including an increased parts count, higher material and assembly costs, and increased size of the current sensor.
A dual totem (or H-bridge) power stage has four power devices (for example four switches, or two switches and two diodes in the case of a two quadrant bridge), at lest some of which are controlled by pulse width modulation (PWM) control signals to change voltage presented across the load with the purpose of affecting the level or polarity of current flowing through the driven load. A current sensor for measuring the load current includes a transformer having a secondary winding having a number N of turns, and a primary winding. The primary winding has four separate primary winding sections each coupled in series with a different one of the four power devices of the dual totem power stage. Different ones of the four primary winding sections have different numbers of turns such that different turns ratios result between the primary winding and the secondary winding for different current paths, thus generating a modulated signal on the secondary winding. Measurement circuitry coupled to the secondary of the transformer demodulates the modulated signal on the secondary winding to provide a load current output indicative of a level of current through the load. Demodulation of the modulated signal occurs by applying different gains to the modulated signal as a function of the turns ratio resulting from a particular current path.